Flue gas recirculation (FGR) is a method used in several industries to control emissions and process conditions, such as NO x reduction and temperature levels, and increase the CO2 concentration in the off-gas, to be better suited for methods of carbon capture. In this study, the influence of FGR, varying levels of flue gas flow and oxygen concentration on the emissions of polycyclic aromatic hydrocarbons (PAHs) was investigated during Si alloy production. In addition, computational fluid dynamics (CFD) modeling was performed using OpenFOAM for combustion of C2H2 and H2 with varying O2 levels to simulate FGR and to gain better insight into the impact of furnace operations on the PAH evolution. Experimental results show that increasing FGR (0–82.5%) and decreasing levels of oxygen (20.7–13.3 vol %) increase the PAH-42 concentration from 14.1 to 559.7 μg/Nm3. This is supported by the simulations, where increased formation of all PAHs species was observed at high levels of FGR, especially for the lighter aromatic species (like benzene and naphthalene), due to the lower availability of oxygen and the reduction in temperature. Residence time was identified as another key parameter to promote complete combustion of PAHs. Benzene oxidation can be prevented with temperatures lower than 1000 K and residence times smaller than 1 s, while complete oxidation is found at temperatures of around 1500 K.

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Coal tar pitch, a well-known source of polycyclic aromatic hydrocarbons (PAHs), is used as a binder of petroleum coke in prebaked anodes used for electrolysis of aluminum. Anodes are baked up to 1100 °C over a 20-day period, where flue gas containing PAHs and volatile organic compounds (VOCs) are treated using techniques such as regenerative thermal oxidation, quenching, and washing. Conditions during baking facilitate incomplete combustion of PAHs, and due to the various structures and properties of PAHs, the effect of temperature up to 750 °C and various atmospheres during pyrolysis and combustion were tested. PAH emissions from green anode paste (GAP) dominate in the temperature interval of 251−500 °C, where PAH species of 4− 6 rings make up the majority of the emission profile. During pyrolysis in argon atmosphere, a total of 1645 μg EPA-16 PAHs are emitted per gram of GAP. Adding 5 and 10% CO 2 to the inert atmosphere does not seem to affect the PAH emission level significantly, at 1547 and 1666 μg/g, respectively. When adding oxygen, concentrations decreased to 569 μg/g and 417 μg/g for 5% and 10% O 2 , respectively, corresponding to a 65% and 75% decrease in emission.